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Abstract:

Method for determining the position and/or displacement of a mobile
element with respect to a fixed frame, including using a fixed light
source emitting a light beam, arranging the source with respect to the
mobile element and a sensor to induce an interaction between the beam and
sensor, includes using a concave mirror, integral in movement with the
mobile element, for reflecting the beam in direction to the sensor,
arranging on the path of the beam a fixed optical mask which presents a
two dimensional regular pattern interlaced with an absolute code,
detecting and processing the image casted by the mask on the sensor,
computing the displacement value of the image on the sensor and using the
computed displacement value for computing and providing the position
and/or the displacement in at least one direction of the mobile element
in dependence of the image's displacement.

Claims:

1. A measurement method for determining position and/or displacement of a
mobile element with respect to a fixed frame, wherein a fixed light
source (1) emitting a light beam is used, the light source (1) is
arranged with respect to the mobile element and with respect to a sensor
(2) in a way so as to induce an interaction between the light beam and
the sensor (2) which depends on the position of the mobile element and
wherein processing and computing means are used for providing a value of
the position and/or the displacement of the mobile element in dependence
of the output signal of the sensor (2), said measurement method
comprising the steps of: a) using a concave mirror (3), fixed to the
mobile element, for reflecting the light beam in direction to the sensor
(2), b) arranging on the path of the light beam a fixed optical mask (4)
which comprises a two dimensional regular pattern, c) detecting and
processing a displacing image casted by the optical mask (4) on the
sensor (2), d) computing the displacement value of said displacing image
on said sensor (2), and e) using the computed displacement value for
computing and providing position and/or displacement in at least one
direction of the mobile element in dependence of displacing image's
displacement.

2. The method of claim 1, wherein the shadow of the optical mask (4)
casted on the sensor (2) is used as the displacing image.

3. The method of claim 1, wherein the displacement of the mobile element
is given by: Δzled=C×Δzmirror where C is a
multiplying value, Δzmirror is the displacement of the concave
mirror (3) in the z direction and Δzled is the displacement of
the virtual position of the light source (1) in the z direction which is
proportional to the displacement of the image casted on the sensor (2).

4. The method of claim 3, consisting in determining the multiplying value
C by using an empirical method.

5. The method of claim 2, wherein the displacement of the mobile element
is given by: Δzled=C×Δzmirror where C is a
multiplying value, Δzmirror is the displacement of the concave
mirror (3) in the z direction and Δzled is the displacement of
the virtual position of the light source (1) in the z direction which is
proportional to the displacement of the image casted on the sensor (2).

6. The method of claim 5, consisting in determining the multiplying value
C by using an empirical method.

7. The method of claim 3, comprising the step of determining
displacements in the z direction which extends orthogonally to the plane
of the sensor (2).

8. The method of claim 5, comprising the step of determining
displacements in the z direction which extends orthogonally to the plane
of the sensor (2).

9. The method according to claim 3, comprising a step of determining
displacements in at least one direction extending in plane which is
parallel to the plane of the sensor (2).

10. A measurement system for determining the position and/or the
displacement of a mobile element with respect to a fixed frame, said
measurement system comprising a fixed light source (1) emitting a light
beam, a concave mirror (3) fixed to the mobile element and arranged to
reflect the emitted light beam toward a fixed sensor (2), a fixed optical
mask (4) arranged on the path of the light beam for influencing said
light beam in dependence of the position of the mobile element and
processing and computing means receiving the output signal of the sensor
(2), for providing the position and/or the displacement of the mobile
element, wherein the optical mask (4) comprises a two dimensional regular
pattern designed to cast an image or a shadow of said optical mask (4) on
the sensor (2), said processing and computing means being designed to
determine the displacement and/or the position of the mobile element from
the displacement of the image casted on the sensor (2) detected by the
sensor.

11. The measurement system of claim 10, wherein the optical mask
comprises a distinctive element in the two dimensional regular pattern.

12. The measurement system of claim 10, where the regular pattern
comprises an absolute code interlaced with the two dimensional regular
pattern.

13. The measurement system of claim 10, wherein the sensor (2) is an
imaging device comprising a pixel array.

14. The measurement system of claim 11, wherein the sensor (2) is an
imaging device comprising a pixel array.

15. The measurement system of claim 12, wherein the sensor (2) is an
imaging device comprising a pixel array.

16. The measurement system according to claim 10, characterized in that
the imaging device comprises a plurality of pixels arranged in two
dimensions and in that the processing and computing means are designed to
compute displacements of the image in two directions defining the imaging
device.

17. The measurement system according to claim 10, wherein the light
source (1) is a punctual light source of the LED kind.

18. The measurement system according to claim 10, wherein the concave
mirror (3) has a spherical or parabolic shape.

19. The measurement system according to claim 10, wherein the optical
mask (4) is a grating comprising transparent and opaque areas for shaping
the two dimensional regular pattern and the absolute code.

20. The measurement system of claim 19, wherein the transparent areas are
made of perforations or holes.

21. The measurement system according to claim 10, characterized in that
the mobile element is a trigger stylus, a touch trigger stylus, a force
measurement system or an accelerometer.

Description:

FIELD OF THE INVENTION

[0001] The present invention relates to the field of absolute positioning
devices or systems, in particular to the field of three or more degrees
of freedom measurement systems such as multi-dimensional position
encoders. Examples of such devices are pointing devices for computers,
measuring devices for tooling, trigger stylus, touch trigger probes or
scanning probes.

[0002] Particularly, the present invention relates to the field of
absolute positioning devices where the measured position ranges from a
few nanometers to a few meters. It relates to positioning devices
associated to measurement systems that measure with a very high accuracy
the position and/or the displacement of a mobile element in space.

[0003] The invention concerns in particular measurement systems which are
able to detect displacements in a very small volume.

BACKGROUND OF THE INVENTION

[0004] Positioning devices are well known in the art, and are used across
several technical domains. In the metrology domain, positioning devices
are mostly found as rotary encoders, as in WO 2006107363 A1, or linear
encoders as in U.S. Pat. No. 5,563,408. These encoders output one
dimensional information about the position, and are operating with a
resolution of the order of 1/10 of a micron or of a 1/10,000 of a degree.
To reach a positioning with several degrees of freedom, these encoders
can be part of a chain, for example in a robotic arm, with the
disadvantage that the more encoders are used, the more the positioning
resolution degrades. The state of the art of robotic arm positioning
system has today a resolution, which reaches at best one micron.

[0005] In a different technical field, the document EP 0 390 648 A1
discloses a multidirectional contact sensor which comprises a mobile
probe. The mobile probe is mounted on a rigid frame with the help of a
flexible membrane. The rear part of the probe comprises a concave mirror
which faces an optoelectronic system comprising optic fibres for
detecting a movement of the probe. The concave mirror reflects a light
beam emitted by the optoelectronic system toward a sensor which detects
the intensity of the reflected light beam. The detected intensity depends
on the orientation of the reflected light beam and though of the position
of the probe. The reaching of different intensity levels is then used for
the control of functions which depend on the position of the probe. Such
a multidirectional contact sensor doesn't fit in a very small volume.
Additionally, the accuracy of the detected positions lies between 0.2 and
0.4 microns which is not satisfactory for very small systems. Intensity
fluctuations may also occur due to fluctuation in power supply,
temperature variation, ageing of the light source, changes of mirror
reflectivity, which impose limits in the stability and accuracy of the
sensor.

SUMMARY OF THE INVENTION

[0006] An object of the present invention is to alleviate the limitation
of the prior art by providing a novel measurement system and a novel
measurement method which allow a very fast and reliable detection and
measurement of nanometric displacements, for instance three dimensional
displacements.

[0007] Another object of the invention is to propose a novel measurement
system and a novel measurement method which allow the detection and
measurement in a very small volume, for instance less than 1 cm3,
with a limited power and a low thermal dissipation, for instance less
than 100 mW.

[0008] Another object of the invention is to propose a novel measurement
system and a measurement method which enhance the resolution, the speed
and the precision of optical position encoders.

[0009] The objects given to the invention are achieved with the help of a
measurement method for determining the position and/or the displacement
of a mobile element with respect to a fixed frame, wherein one uses fixed
light source emitting a light beam, one arranges the light source with
respect to the mobile element and with respect to a sensor in a way so as
to induce an interaction between the light beam and the sensor which
depends on the position of the mobile element and one uses processing and
computing means for providing a value of the position and/or the
displacement of the mobile element in dependence of the output signal of
the sensor. According to the invention, the method comprises the steps
of:

[0010] using a concave mirror, fixed to the mobile element, for
reflecting the light beam in direction to the sensor,

[0011] arranging on
the path of the light beam a fixed optical mask which comprises a two
dimensional regular pattern,

[0012] detecting and processing the image
casted by the optical mask on the sensor,

[0013] computing the
displacement value of said image on said sensor, and

[0014] using the
computed displacement value for computing and providing the position
and/or the displacement in at least one direction of the mobile element
in dependence of the image's displacement.

[0015] The displacement value of said image on the sensor is proportional
to the displacement of the virtual position of the light source.

[0016] In an implementation in accordance with the invention, the method
comprises a step of determining displacements in at least one direction
extending in plane which is parallel to the plane of the sensor.

[0017] In an implementation in accordance with the invention, the method
consists in using the shadow of the optical mask casted on the sensor, as
the displacing image.

[0018] In an implementation in accordance with the invention, the
displacement of the mobile element is given by:

Δzled=C×Δzmirror

where C is a multiplying value, Δzmirror is the displacement
of the concave mirror in the z direction and Δzled is the
displacement of the virtual position of the light source in the z
direction which corresponds to the displacement of the image casted on
the sensor.

[0019] In an implementation in accordance with the invention, the method
consists in determining the multiplying value C by using an empirical
method.

[0020] In an implementation in accordance with the invention, the method
comprises the step of determining displacements in the z direction which
extends orthogonally to the plane of the sensor.

[0021] The objects given to the invention are achieved with the help of a
measurement system for determining the position and/or the displacement
of a mobile element with respect to a fixed frame, comprising a fixed
light source emitting a light beam, a concave mirror fixed to the mobile
element and arranged to reflect the emitted light beam toward a fixed
sensor, a fixed optical mask arranged on the path of the light beam for
influencing said light beam in dependence of the position of the mobile
element and processing and computing means fed with the output signal of
the sensor, for providing the position and/or the displacement of the
mobile element, characterized in that the optical mask comprises a two
dimensional regular pattern designed to cast an image or a shadow of said
optical mask on the sensor, the processing and computing means being
designed to determine the displacement and/or the position of the mobile
element from the detected displacement of the image casted on the sensor.

[0022] In an embodiment in accordance with the invention, the optical mask
comprises a distinctive element in the regular pattern.

[0023] In an embodiment in accordance with the invention, the regular
pattern comprises an absolute code interlaced with the regular pattern.

[0024] In an embodiment in accordance with the invention, the sensor is an
imaging device comprising a pixel array.

[0025] In an embodiment in accordance with the invention, the imaging
device comprises a plurality of pixels arranged in two dimensions and in
that the processing and computing means are designed to compute
displacements of the image in two directions defining the imaging device.

[0026] In an embodiment in accordance with the invention, the light source
is a punctual light source of the LED kind.

[0027] In an embodiment in accordance with the invention, the concave
mirror has a spherical or parabolic shape.

[0028] In an embodiment in accordance with the invention, the optical mask
is a grating comprising transparent and opaque areas for shaping the two
dimensional regular pattern and the absolute code.

[0029] In an embodiment in accordance with the invention, the transparent
areas are made of perforations or holes.

[0030] In an embodiment in accordance with the invention, the mobile
element is a trigger stylus, a touch trigger stylus, a force measurement
system or an accelerometer.

[0031] An advantage of the method and of the system in accordance with the
invention lies in the fact that it decreases the smallest detectable
displacement by optical magnification.

[0032] The measurement system in accordance with the invention can detect
displacements as low as 1 to 5 nanometers and this with computing time in
the order of 1 microsecond.

[0033] Another advantage of the method and of the system in accordance
with the invention lies in the fact that it decreases by a factor of 10
the number of photons needed for the detection of nanometric
displacements. The current for the light source can so be reduced. This
permits to achieve a reduction of the power consumption and of the heat
dissipation of the measurement system. The reduced heat dissipation
increases the thermal stability, accuracy and reliability.

BRIEF DESCRIPTION OF THE DRAWINGS

[0034] The invention will be better understood by reading the following
description, provided in reference to the annexed drawings where:

[0035] FIG. 1, shows the functional principle of an embodiment of the
measurement system in accordance with the invention,

[0036] and FIG. 2, illustrates an advantage obtained with the measurement
system of FIG. 1.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

[0037] Elements that are structurally and functionally identical, and that
are present in more than one distinct figure or illustration, are given
the same numeric or alphanumeric reference in each of them.

[0038] In the following description, we will first present the measurement
system, as illustrated in FIG. 1, which comprises a single point light
source 1, a sensor 2, a concave mirror 3 and an optical mask 4 arranged
to cast an image and a shadow of said optical mask 4, on the sensor 2.

[0039] The sensor 2 is a two dimensional imaging device comprising a pixel
array associated to processing and computing means. The pixel array is
preferably constituted of an assembly of photodiodes. The imaging device
comprises a plurality of pixels arranged in two dimensions and the
processing and computing means are designed to compute displacements of
the image or shadow, casted on the imaging device, at least in two
directions.

[0040] The optical mask 4, is a fixed grating which comprises a two
dimensional regular pattern designed to cast an image and a shadow of
said optical mask 4 on the sensor 2. To handle a positioning measure,
which can vary more than the distance between two neighboring elements
forming the regular pattern, a distinctive element can be added to the
regular pattern. The distinctive element can be a missing element of the
regular pattern, or a missing row and column of the regular pattern. The
distinctive element can also be embodied by the end, i.e. the border, of
the regular pattern, provided that the border is visible on the sensor.

[0041] In another embodiment in accordance with the invention, in
particular for larger displacements of the mobile element, the optical
mask 4 comprises an absolute code interlaced with the regular pattern.

[0042] The point light source 1 is an immobile light source emitting a
light beam toward the concave mirror 3. The point light source 1 is
preferably a punctual light source of the LED kind.

[0043] The concave mirror 3 presents a curvature preferably comprised
within the range 5 to 20 mm. The concave mirror 3 has for instance a
parabolic or spherical shape.

[0044] The concave mirror 3 is fixed to the mobile element and arranged to
reflect the emitted light beam toward the fixed sensor 2. The fixed
optical mask 4 is arranged on the path of the light beam for influencing
said light beam in dependence of the position and/or the displacements of
the mobile element.

[0045] The processing and computing means are designed to determine by
using the detected displacement of the image or shadow casted on the
sensor 2, the displacement and/or the position of the mobile element.

[0046] As an example, the optical mask 4 can be made with a
chromium-plated glass. The light is blocked at the locations where
chromium is deposited, and can go through the glass elsewhere. The
preferred embodiment is the one using opaque regions and holes for
implementing transparent regions. For example a grating made of nickel
and holes may be used. Today Nickel plates can be manufactured at low
cost, with thicknesses less than 50 microns, and with an accuracy of the
holes of one micron over a couple centimetres. It is preferred to
implement transparent regions by holes instead of by glass, because the
light goes straight through the holes, while it is slightly deviated by a
glass layer, according to Snell's law.

[0047] An example of an optical mask 4 or grating that uses a
two-dimensional code as distinctive element, which is interlaced with the
repetitive patterns, is described in EP 2 169 357 A1.

[0048] In another embodiment of the invention, the optical mask 4 can be
implemented by using a microlens array. In other words, the component
pattern is a microlens and the distinctive element is a missing microlens
region. Each black dot represents the position of a micro-lens. The
microlenses are more expensive to produce than a conventional grating,
but generate a shadow pattern, which has more light, and thus allows for
a faster measurement system. In addition, the diffraction phenomena, also
known as Talbot effect, have a substantially smaller influence on the
shadow pattern. This last advantage allows for more flexibility in the
choice of the distance between the element and the imaging device.

[0049] In another embodiment of the invention, the point light source 1 is
connected to the computing means and to the imaging device. By connected,
we mean that there is at least one electrical connection between the
computing means, the imaging device and the point light source 1. For
example, the point light source 1 can be placed next to the imaging
device on the same circuit, or even in the middle of the imaging device.
This configuration requires only one power supply, and allows for a very
convenient synchronisation between the image capture and the light
emission.

[0050] The measurement system enables to carry out a measurement method as
described below.

[0051] The measurement method, for determining the position and/or the
displacement of a mobile element with respect to a fixed frame, consists
in using the fixed point light source 1 emitting a light beam and in
arranging the point light source 1 with respect to the mobile element and
with respect to the sensor 2 in a way so as to induce an interaction
between the light beam and the sensor 2 which depends on the position of
the mobile element.

[0052] The measurement method aims to use the processing and computing
means for providing the value of the position and/or the displacement of
the mobile element in dependence of the output signal of the sensor 2.

[0053] In a step a), the measurement method consists in using the concave
mirror 3 which is integral in movement with the mobile element, for
reflecting the light beam in direction to the sensor 2.

[0054] In a step b), the measurement method consists in arranging on the
path of the light beam the fixed optical mask 4 which presents a two
dimensional regular pattern interlaced with an absolute code.

[0055] In a step c), the measurement method consists in detecting and
processing the image casted by the optical mask 4, on the sensor 2.

[0056] In a step d), the measurement method consists in computing the
displacement value of said image or shadow on said sensor 2.

[0057] In a step e), the measurement method consists in using the computed
displacement value for computing and providing the position and/or the
displacement in at least one direction of the mobile element in
dependence of the image's or of the shadow's displacement.

[0058] In an implementation in accordance with the invention, the
measurement method consists in using the shadow of the optical mask 4
casted on the sensor 2, as the displacing image.

[0059] When assuming of small angles (R>>y) leading to corresponding
geometrical approximations and that an origin is defined on the position
of the light source 1, the virtual position of the light source is given
by:

zled=2×zm×(R-zm)/(R-2zm)

where R is equal to 2f, f being the focal distance of the concave mirror
3, zm being the position of the concave mirror 3 and R>>y, y
being the assumed position of the light ray on the concave mirror 3 with
respect of his centre.

[0060] In an implementation in accordance with the invention in which the
concave mirror 3 moves together with the mobile element in the orthogonal
direction Z with respect to the sensor 2, the displacement of said mobile
element is given by:

Δzled=C×Δzmirror

where C is a multiplying value, Δzmirror is the displacement
of the concave mirror 3 in the z direction and Δzled is the
displacement of the virtual position of the light source 1 in the Z
direction which is proportional to the displacement of the image casted
on the sensor 2.

[0061] The multiplying value C may be a given constant value for a
displacement of the concave mirror 3 between two given successive
positions. In some applications, C remains quasi-constant in a small
range of angles and mirror positions. For example for touch trigger
probes the displacements of the moving part are in the order of
micrometers and C can be considered constant.

[0062] In another implementation in accordance with the invention, the
measurement method consists in determining displacements in at least one
additional direction, extending in plane which is parallel to the plane
of the sensor 2.

[0063] FIG. 1 illustrates an implementation of the method in accordance
with the invention. The displacement dz of the concave mirror 3, from the
position 3b to the position 3a induces the displacement dvs of the
virtual positions of the point light source 1, from position 1b to
position 1a.

[0064] It appears that this displacement dvs, which is detected on the
imaging device and which is more important than the real displacement of
the mobile element or the concave mirror 3, enhances the resolution of
the measurement system. The measurement system in accordance with the
invention enables to amplify the detected displacements and at the same
time it improves its efficiency by concentrating the light on the sensor
2 as it is illustrated in FIG. 2.

[0065] The concave mirror 3 concentrates the light of the point light
source 1 (LED) to the sensor 2, resulting in a decrease of the needed
amount of photons generated by the LED for the detection by roughly a
factor 10. Without the concave mirror 3, the virtual point light source
1a would emit the light in the virtual solid angle α while the use
of the concave mirror 3 reduces the solid angle to β. The optical
efficiency is therefore increased.

[0066] This description has been provided only for purpose of non limiting
example. Those skilled in the art may adapt the invention but keeping
within the scope of the invention as defined in the claims. Naturally, it
is possible to envisage replacing any of the means described or any of
the steps described, with equivalent means or an equivalent step without
going beyond the scope of the present invention.